Elsevier

Journal of Catalysis

Volume 216, Issues 1–2, May–June 2003, Pages 298-312
Journal of Catalysis

State of the art and future challenges of zeolites as catalysts

https://doi.org/10.1016/S0021-9517(02)00132-XGet rights and content

Abstract

The control of pore diameter and topology of zeolites, as well as the nature of active sites and adsorption properties, allow in many cases the a priori design of catalysts for applications in the fields of oil refining, petrochemistry, and the production of chemicals and fine chemicals. The potentiality of nanocrystalline, delaminated, or ultralarge pore catalysts and of zeolites formed by channels with different dimensions is outlined.

Introduction

Zeolites are crystalline silicates and aluminosilicates linked through oxygen atoms, producing a three-dimensional network containing channels and cavities of molecular dimensions. Crystalline structures of the zeolite type but with coordinated Si, Al, or P as well as transition metals and many group elements such as B, Ga, Fe, Cr, Ge, Ti, V, Mn, Co, Zn, Be, Cu, etc. can also by synthesized, and they are referred by the generic name of zeotypes; they include, among others, ALPO4, SAPO, MeAPO, and MeAPSO molecular sieves [1], [2], [3], [4], [5].

Such tridimensional networks of well-defined micropores can act as reaction channels whose activity and selectivity will be enhanced by introducing active sites. The presence of strong electric fields and controllable adsorption properties within the pores will produce a unique type of catalyst, which by itself can be considered as a catalytic microreactor. Summarizing, zeolites are solid catalysts with the following properties:

  • High surface area.

  • Molecular dimensions of the pores.

  • High adsorption capacity.

  • Partitioning of reactant/products.

  • Possibility of modulating the electronic properties of the active sites.

  • Possibility for preactivating the molecules when in the pores by strong electric fields and molecular confinement.

If the accumulation of knowledge allows us now to see many catalytic possibilities for zeolites, the beginnings in this field were much more limited. Indeed, the two first properties outlined above, i.e., high surface area and molecular dimensions of the pores, were early recognized by Barrer [6], [7], who applied them to the separation of linear and branched hydrocarbons. Thus, Union Carbide invested heavily in fundamental research on zeolite synthesis and separation of molecules and the Linde Division developed in 1948 molecular sieve commercial adsorbents based on the synthetic aluminosilicates zeolites A and X [8], [9]. Very soon, Rabo and his group at Union Carbide envisaged the possibilities of zeolites as catalysts by introducing acid sites and rationalizing that the interaction between acid sites and reactant molecules involved not only the protic sites but also the adsorption of the molecule onto the surrounding zeolite crystals [10]. These studies opened the door for perhaps the biggest revolution in oil refining, the introduction of acid zeolite Y as a commercial FCC catalyst by Mobil (today ExxonMobil) [11].

The different features of zeolites that make these catalysts unique will be discussed.

Section snippets

Shape selectivity control

Analogously to enzymes, zeolites with their regular well-defined pore dimensions are able to discriminate [12] reactants and products by size and shape when they present significant differences in diffusivity through a given pore channel system. A particular relevant example of this is the selective cracking of n-paraffins and n-olefins with respect to their branched isomers using medium-pore-size zeolites with pore diameters in the range 0.45–0.56 nm. This effect is based on zeolite shape

Control of adsorption properties

Enzymes are also able to select reactants and products by polarity and in other cases can perform bimolecular reactions between two reactants with different polarities. It should also be emphasized that enzymes are able to work in aqueous media but the adsorption of water can be controlled. Thus, using the enzymatic model, the possibilities of zeolites as catalysts could be improved if the adsorption properties could be adjusted by either selecting an adequate solvent or, even better,

Activating the reactants by confinement effects in zeolites

When a molecule is confined in the pores of a zeolite, the sorption energy will include different energy terms E=ED+ER+EP+EN+EQ+EI+EAB, where ED and ER are the attractive and repulsive contribution terms, respectively, from the van der Waals interaction; EP, EN, and EQ are the polar, field-dipole, and field gradient-quadrupole terms, respectively; EI is the sorbate–sorbate intermolecular interaction energy, and EAB is the energy of the intrinsic acid–base chemical interaction. One can safely

Catalytic acid sites in zeolites

To summarize all the relevant work done in this field in a few pages has become an impossible task for us. Nevertheless, we will try to emphasize some published work that illustrates the possibilities of zeolites as acid catalysts.

Brønsted acid sites are generated on the surfaces of zeolites when Si4+ is isomorphically replaced by a trivalent metal cation such as, for instance, Al3+. This substitution creates a negative charge in the lattice that can be compensated by a proton. From a

Future perspectives in zeolite acid catalysts

We believe that one has to look at acid zeolites from the point not only of view of their intrinsic acidities, but the role played by the short- and medium-long range effects on adsorption and stabilization of the activated complex should also be considered. It seems logical that the structure will determine the spatial conformation as well as the number of hydrogen bonds that the “protonated transition complex” can form with the framework anion in order to get the minimum energy configuration.

Zeolites with basic active sites

It is also possible to generate basic sites within the pores of zeolites and in this way to take advantage of the properties of zeolites in base catalysis. In the case of zeolites the basic sites are of Lewis type and correspond to framework oxygens, and the basicity of a given oxygen will be related to the density of negative charge. Taking this into account, the basicity will be a function of framework composition, the nature of extraframework cations, and the zeolite structure.

Future trends on basic catalysis in zeolites

By generating framework and/or extraframework basic sites, it is now possible to prepare zeolites within a very large spectrum of basicities. Then, depending on the reaction to be catalyzed, it should be possible to select the most adequate basic zeolite from the very mild alkaline-exchanged zeolites up to very strong alkali- or alkaline-oxide-cluster containing zeolites. In principle, basic zeolite catalysts should be available for any of the following base-catalyzed reactions: olefin

Zeolites with redox active sites

Many oxidation processes in the liquid phase are catalyzed by soluble oxometalic compounds. These catalysts present two main limitations. One is the tendency of some oxometalic species to oligomerize, forming μ-oxocomplexes that are catalytically inactive. Another limitation is the oxidative destruction of the ligands that lead to the destruction of the catalysts. Solving these two problems will require isolating the catalytically active sites on inorganic matrices through supporting metals,

N2O as oxidant

Dehydroxylated and high-silica ZSM-5 zeolites have been used as catalysts for the selective oxidation of aromatic compounds including benzene, chlorobenzene, difluorobenzenes, phenol, styrene, and alkylbenzenes to their corresponding phenol derivatives, using nitrous oxide as oxidant [172], [173]. During the steaming of HZSM-5, strong Lewis acid–base pair sites are formed and they were able to hydroxylate benzene with N2O, producing high yields of phenol (70–80%) with high selectivity and

Conclusions and perspectives in catalysis

Zeolites have been shown to be useful catalysts in a large variety of reactions, from acid to base and redox catalysis. We have seen that they will offer new opportunities for reactions in the field of chemical and fine chemicals if, besides the nature of active sites and dimensions and shape of the pores, one is able to tune the adsorption properties and local geometry of the active sites. However, for many important applications, the size of the zeolitic pores are too small to react the bulky

Acknowledgements

Financial support by the Spanish CICYT (MAT2000-1392) is gratefully acknowledged.

References (197)

  • J. Weitkamp

    Solid State Ionics

    (2000)
  • N.Y. Chen et al.

    J. Catal.

    (1978)
  • S.J. Miller

    Micropor. Mater.

    (1994)
  • W.J. Souverijns et al.

    Stud. Surf. Sci. Catal.

    (1997)
  • Th.L. Maesen et al.

    J. Catal.

    (1999)
  • G. Sastre et al.

    J. Catal.

    (2000)
  • S.J. Chu et al.

    Appl. Catal. A

    (1995)
  • J.A. Horsley et al.

    J. Catal.

    (1994)
  • G. Pazzuconi et al.

    Stud. Surf. Sci. Catal.

    (2001)
  • P. Andy et al.

    J. Catal.

    (2000)
  • M.A. Camblor et al.

    J. Catal.

    (1998)
  • P. Botella et al.

    J. Catal.

    (2001)
  • S.M. Csicsery

    J. Catal.

    (1970)
  • S.M. Csicsery

    J. Catal.

    (1971)
  • S.M. Csicsery

    Zeolites

    (1984)
    S.M. Csicsery

    J. Catal.

    (1987)
  • J.C. Van der Waal et al.

    Catal. Lett.

    (1996)
  • S. Namba et al.
  • H. Ogawa et al.

    J. Catal.

    (1994)
  • A. Corma et al.

    Stud. Surf. Sci. Catal.

    (2001)
  • M.A. Camblor et al.

    J. Catal.

    (1997)
  • M.J. Climent et al.

    J. Catal.

    (2000)
  • E.G. Derouane

    J. Catal.

    (1986)
  • E.G. Derouane

    Chem. Phys. Lett.

    (1987)
  • E.G. Derouane et al.

    J. Catal.

    (1988)
  • A. Corma et al.

    J. Phys. Chem.

    (1994)
  • J. Sauer

    Chem. Rev.

    (1989)
  • W.D. Haag

    Stud. Surf. Sci. Catal.

    (1994)
  • L.A. Pine et al.

    J. Catal.

    (1984)
  • D. Barthomeuf

    Mater. Chem. Phys.

    (1987)
  • E. Armengol et al.

    Appl. Catal. A

    (1995)
  • R. Fisher, W. Hölderich, W.D. Mrooz, M. Srohmeyer, Eur. Patent 0167021,...
  • G.P. Heitmann et al.

    J. Catal.

    (1999)
  • N.S. Gnep et al.

    Bull. Soc. Chim. Fr.

    (1977)
  • L.D. Fernandes et al.

    J. Catal.

    (1998)
  • M. Guisnet et al.

    Micropor. Mesopor. Mater.

    (2000)
  • P. Sarv et al.

    J. Phys. Chem.

    (1998)
  • D. Barthomeuf

    Stud. Surf. Sci. Catal.

    (1978)
  • H. Knözinger et al.

    J. Chem. Soc. Faraday Trans.

    (1998)
  • M. Sánchez-Sánchez et al.

    Chemm. Commun.

    (2000)
  • L. Wytterhoeven et al.

    J. Chem. Soc. Faraday Trans.

    (1992)
  • A. Corma et al.

    J. Catal.

    (1991)
  • E.M. Flanigen

    Stud. Surf. Sci. Catal.

    (1991)
  • Q. Huo et al.

    J. Chem. Soc. Chem. Commun.

    (1992)
  • M. Estermann et al.

    Nature

    (1991)
  • M.J. Annen et al.

    J. Chem. Soc. Chem. Commun.

    (1991)
  • D.E.W. Vaughan, US Patent 5,976,491,...
  • R.M. Barrer et al.

    Trans. Faraday Soc.

    (1944)
  • R.M. Barrer, British Patent 574911,...
  • D.W. Breck

    J. Chem. Education

    (1964)
  • R.M. Milton
  • Cited by (0)

    View full text